When an object is thrown straight up, it initially accelerates upward as it gains speed. Once it reaches the very top of its path, it stops accelerating and begins to decelerate due to gravity, until it reaches its maximum height and starts to fall back down.
At the very top of its path, the acceleration of the red ball is zero, because it is no longer gaining speed, it has reached its maximum height and is starting to fall back down.
The acceleration of the red ball at the very top of its path is zero m/s^2.
It's important to note that the initial velocity is 7m/s and the acceleration is the change of velocity over time, which is equal to zero because the velocity is the same and hasn't changed, the ball is at the highest point and it is not moving anymore.